In multi-link transmission, a single data stream is partitioned over a number of parallel links between a transmitter and a receiver. In Asynchronous Transfer Mode (ATM) networks, for example, a multi-link transmission technique known as inverse multiplexing is used. This technique is described in document AF-PHY-0086.001, promulgated by the ATM Forum (1999), entitled “Inverse Multiplexing for ATM (IMA) Specification Version 1.1,” which is incorporated herein by reference. IMA allows data to be transferred to and from one virtual ATM port by multiplexing over a number of parallel physical, point-to-point lines, such as E1 (2.048 Mbps) or T1 (1.544 Mbps) lines. Up to 32 such lines may be used, as stated in the IMA standard, although it is generally economical to use no more than eight lines, with the actual number of lines depending on the required bandwidth. IMA is typically used to serve sites for which a single E1 or T1 line does not give sufficient bandwidth, while a high-rate interface is not needed or economically justified.
Multi-link methods are also known in the Digital Subscriber Line (DSL) field. DSL is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire, which is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access high-speed digital networks, such as ATM and Internet Protocol (IP) networks, without requiring major investments in new infrastructure. A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation.
Very high speed DSL (VDSL) access transmission systems, for example, are described in standard TS 101 270-2 V1.1.3 (2000-09) of the Transmission and Multiplexing (TM) Technical Committee of the European Telecommunications Standards Institute (ETSI), entitled “Access Transmission Systems on Metallic Access Cables; Very High Speed Digital Subscriber Line (VDSL),” which is incorporated herein by reference. According to this standard (section 5.4), VDSL transceivers can be configured to carry two parallel subchannels over the same wire pair. Each subchannel corresponds to a different band of frequencies. In current VDSL modems, the data rates of the two channels are both integer multiples of a basic symbol rate BSR, i.e., the data rate of one channel is R1=N1*BSR, and that of the other channel is R2=N2*BSR, so that the ratio of the data rates between the two subchannels is N1:N2. In a multi-band configuration, data should clearly be split between the two channels in this same ratio in order to use the channel resources efficiently.
In the approach currently defined by VDSL standards, the N1:N2 ratio between the channels is maintained by alternately routing N1 bytes for transmission on channel 1, and then routing N2 bytes for transmission on channel 2. This approach has some disadvantages that stem from the fact that the values of N1 and N2 can be up to several hundred. Thus, for example, 200:201 is a valid N1:N2 ratio. In this case, the data are split at the transmitter in the following way:                Forward 200 bytes to channel 1        Forward 201 bytes to channel 2        Forward 200 bytes to channel 1        Forward 201 bytes to channel 2        Forward 200 bytes to channel 1        Forward 201 bytes to channel 2        . . .        
It can be seen that this block-based splitting scheme does not give good interleaving of bytes between the two subchannels. Therefore, if one of the subchannels has a lower noise margin than the other (resulting in a higher incidence of bit errors), the result will be bursts of errors coming from this subchannel in the output data stream at the receiver. A further disadvantage of this implementation is the need for big input buffers to accumulate at least N1 and N2 bytes at both the transmitter and the receiver. The additional buffering also results in added latency.
Another type of DSL access system capable of multi-link operation is Symmetrical high-speed DSL (SDSL), as described in ETSI standard TS 101 524 V1.1.2 (2001-08), entitled “Transmission and Multiplexing (TM); Access Transmission Systems on Metallic Access Cables; Symmetrical Single Pair High Bitrate Digital Subscriber Line (SDSL),” which is incorporated herein by reference. SDSL can also operate in a four-wire mode, in which the transmitter and receiver are connected by two pairs of wires. This implementation is described by Leshem in “Multichannel SDSL Optional Mode,” published as ETSI document TM6 WD19 (Stockholm, Sweden, September, 2001) which is incorporated herein by reference. For pairs 1 and 2 having respective rates R1 and R2, Leshem proposes that the first L1 bytes in each block be allocated to pair 1, and the remaining L2 bytes be allocated to pair 2, wherein L1/L2=R1/R2.
A number of proposals have been made for improving the transport of data streams over multi-link SDSL connections. For example, Volkening et al. suggest that IMA be used for carrying ATM streams over SDSL in “IMA Support in SDSL Access Environment,” published as ETSI document TM6 TD 39 (Stockholm, Sweden, September, 2001), which is incorporated herein by reference. This proposal is based on the SDSL four-wire mode, in which the transmitter and receiver are connected by two physical links. In the IMA mode, the transmitter multiplexes the incoming ATM cells alternately, cell by cell, over the two links. Although relatively easy to implement, this solution is limited to ATM applications and does not address the needs of other types of data streams. Furthermore, because the minimum block size for IMA is a single ATM cell (53 bytes), this approach suffers from limitations of poor interleaving and added latency in the data transport. The cells are divided equally between the two links, with no provision for possible differences in data rate due to different noise margins. Therefore, the rate of data transport is limited by the rate of the worst-case link.
Leshem suggests that multiple different constellation sizes be used for multi-pair SDSL transmission, in “Constellations and Framing in Two Pairs SDSL,” published as ETSI document TM6 TD 58 (Stockholm, Sweden, September, 2001), which is incorporated herein by reference. The constellation for each pair is adapted to give higher or lower bit loading per symbol, depending on the signal conditions on that pair. In this way, the overall data carrying capacity of the multiplexed links is increased, while maintaining the same baud rate on all the links. This method adds complication, however, in the physical-layer processing of the transmitter and receiver, which must be configured to handle the variable constellations, while providing only coarse control (in 6 dB steps) of the rate selection on each channel. It also fails to solve the above-mentioned problems of poor interleaving and high latency.